The present invention relates to an image forming apparatus for forming an image by transferring a color medium from a transfer medium to a sheet of paper in accordance with an image pattern.
Among the known color image forming apparatus of this type is a thermal head printer in which different colored inks are melted in accordance with an image pattern and then are transferred to a sheet of paper.
Small, low-priced, noise-free and capable of printing on ordinary paper, thermal head printers have recently been used as output devices of computers and word processors, and in copying apparatuses.
In a printer of this kind, a sheet of paper to which an image is to be printed is automatically supplied from a paper supply cassette or manually supplied by using a manual feed guide. By operating sheet size selecting keys, an image can be formed on the paper sheet of the selected size in such a manner that the image region is not located beyond the confines of the paper sheet.
However, the size of sheets which the operator can designate is limited to a standard size, such as A4 size and letter size. If a paper sheet of a nonstandard size is used, or if a paper sheet is supplied at an angle by accident, the paper sheet will not conform with the shape or size of an image region to be formed. As a result, part of the image region may be located outside of the paper sheet. Since the image located in such a part is transferred to the platen supporting the paper sheet, the platen may be stained, causing the paper sheet supplied next to be stained.